1. Field of the Invention
The present invention relates to an accumulation type fuel injection system for supplying high-pressure fuel accumulated in a common rail into cylinders of an internal combustion engine by injection performed by fuel injection valves. Specifically, the present invention relates to a structure for assembling a pipe connector formed separately from the common rail to a pipe connecting portion of the common rail.
The present invention also relates to a connection structure for connecting a fuel pipe with the common rail.
2. Description of Related Art
Conventionally, in an accumulation type fuel injection system known as a fuel injection system for a diesel engine, a fuel supply pump pressurizes fuel and pressure-feeds the pressurized fuel to a common rail. The common rail accumulates the high-pressure fuel. The high-pressure fuel accumulated in the common rail is distributed to a plurality of electromagnetic fuel injection valves (injectors) connected to downstream ends of high-pressure pipes branching from the common rail. Then, the high-pressure fuel is supplied by injection from the injectors of respective cylinders into the respective cylinders of the engine.
In this case, as shown in FIGS. 12 and 13, a common rail 200 used in the conventional accumulation type fuel injection system is formed with an accumulation chamber 201 for accumulating the high-pressure fuel. The common rail 200 is formed with a fuel passage hole 202 in a lower side of the accumulation chamber 201 in FIG. 12 in a radial direction substantially perpendicular to an axial direction of the accumulation chamber 201. The common rail 200 is formed with a plurality of fuel passage holes 203 formed in an upper side of the accumulation chamber 201 in FIG. 12 in the radial direction substantially perpendicular to the axial direction of the accumulation chamber 201. An accumulator main body 204 of the common rail 200 is integrated with a pipe connecting portion 205 for connecting the common rail 200 with a high-pressure pipe, which is connected to a fuel supply pump. The accumulator main body 204 is integrated with a plurality of pipe connecting portions 206 for connecting the common rail 200 with high-pressure pipes, which are connected to injectors of respective cylinders.
In a manufacturing process of the common rail 200 used in the conventional accumulation type fuel injection system, first, a material with a low degree of hardness such as low-carbon steel is put into forging dies comprising a pair of an upper die and a lower die engraved with a predetermined shape, and is pressurized. Thus, a forged product having a complete round cylinder portion with a cross section in the shape of a complete round and a plurality of pipe connecting portions integrated with the complete round cylinder portion is formed. Then, the accumulation chamber 201 having a cross section in the shape of a complete round is formed in the accumulator main body 204 by using a cutting tool such as a drill and by combining rotational machining movement and linear feeding movement in an axial direction of the rotational machining movement.
Then, the fuel passage holes 202, 203 having cross sections in the shape of a complete round are formed in the pipe connecting portions 205, 206 respectively by using a machining tool such as a drill and by combining the rotational machining movement and the linear feeding movement in the axial direction of the rotational machining movement. Further, a pressure receiving seat surface 221 is formed at an end of the pipe connecting portion 205 by machining the end of an inner periphery of the fuel passage hole 202 so that an internal diameter of the pressure receiving seat surface 221 is gradually increased outward. Likewise, a pressure receiving seat surface 222 is formed at an end of each pipe connecting portion 206 by machining the end of an inner periphery of the fuel passage hole 203 so that an internal diameter of the pressure receiving seat surface 222 is gradually increased outward. Flange-shaped connection heads formed at ends of the high-pressure pipes adhere to the pressure receiving seat surfaces 221, 222 respectively.
Then, fastening portions 207, 208 are formed by machining outer peripheral surfaces of the ends of the pipe connecting portions 205, 206 with a screwing tool. Thus, the accumulator main body 204 having the cross section in the shape of a complete round and the pipe connecting portions 205, 206 respectively having the cross section in the shape of a complete round are formed by machining the forged product in the predetermined shape as shown in FIGS. 12 and 13. As a result, the cost is increased due to a difficulty in the machining of the outer periphery.
Therefore, a method of forming pipe connectors separately from a common rail, and connecting the pipe connectors to the common rail has been proposed, for instance, in JP-A-10-259772 (pages 3-5, FIG. 2) and JP-A-2001-82663 (page 3, FIG. 1).
In the case where the common rail and the pipe connectors are formed separately, as shown in FIG. 14A, a seal surface 303 at an end of a fuel pipe 302 is liquid-tightly fitted to a pressure receiving seat surface 304 provided at a common rail 301. A fuel passage 307 of the fuel pipe 302 is connected with an accumulation chamber 305 of the common rail 301 via a communication hole 306 intersecting with the accumulation chamber 305. The fuel is accumulated in the accumulation chamber 305 at ultra high pressure of 200 MPa. Therefore, it is required to ensure strength at an intersecting portion between the communication hole 306 and the accumulation chamber 305, while ensuring a machining margin for the pressure receiving seat surface 304. Therefore, a great wall thickness “B” is needed at the intersecting portion of the communication hole 306 and the accumulation chamber 305. Further, sufficient strength is needed also in a connection between a pipe connector 308 and the common rail 301. Therefore, a bonding surface 309 between the pipe connector 308 and the common rail 301 is required to have an area “C” capable of ensuring the strength. As a result, a size “A” of the common rail 301 will be enlarged, that is, an external diameter of the common rail 301 will be increased.
In another proposed method, a connecting surface 404 where a pipe connector 403 is connected to a common rail 401 is deeply machined in order to inhibit the increase in the size of the common rail 401 as shown in FIG. 14B. In this case, a wall thickness “B” of the common rail 401 is increased only at a neighborhood of a pressure receiving seat surface 402, and a bonding area “C” between a bonding surface 404 and the pipe connector 403 is ensured as shown by FIG. 14B. However, in such a method, machining cost is increased.